Big Gunz are Ork field artillery units, commonly crewed by Gretchin.
They are usually large guns mounted in a cannon-style, wheeled for transport and low to the ground so their Gretchin crews can access them. This concept was no doubt copied from encounters with Imperial artillery units, but Big Guns teams are far less disciplined then their Imperial counterparts. Big Gunz are used to pound enemy lines into submission and destroy enemy vehicles from a distance as Boyz close in.
They are often used in groups and have a single team of gretchin crewing up to three of the guns at once under the supervision of a Runtherd. Grots commonly fear the prospect of being included into Big Gun crews and some Runtherds often use the threat of serving in a Big Gunz battery to to keep the rebellious Grots in line.
From Felix’s collection.